1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically, to bidirectional wavelength-division multiplexing (WDM) systems.
2. Description of the Related Art
A unidirectional WDM system uses an optical waveguide, e.g., an optical fiber, to transmit optical signals in one direction. To keep inter-channel crosstalk below a certain threshold, communication channels of such a system have to be appropriately spaced. In general, wider channel spacing corresponds to lower crosstalk, which may limit the number of available communication channels per optical fiber within an allocated wavelength range.
FIG. 1 shows an exemplary bidirectional interleaved WDM system 100 of the prior art. In system 100, optical fibers 110A-C, are configured to transmit optical signals simultaneously in two opposite directions, i.e., West-to-East (W/E) and East-to-West (E/W). System 100 has two transmitter sets 102E-W and two receiver sets 114E-W with a pair of receiver/transmitter sets located at each end (e.g., sets 102W and 114W at the West end) of system 100. Transmitter set 102W is coupled to a multiplexer (MUX) 104W configured to combine signals corresponding to individual transmitter channels of set 102W into a multi-channel signal for W/E transmission along fibers 110 to receiver set 114E by way of demultiplexer (DMUX) 112E. Transmitter set 102E is coupled to MUX 104E to similarly transmit signals in the E/W direction along fibers 110 to receiver set 114W by way of DMUX 112W. Each DMUX 112E-W is configured to separate the received multi-channel signal into different signals corresponding to individual receiver channels of the corresponding receiver set 114E-W.
System 100 of FIG. 1 has three fiber spans 110A-C, although a different number of spans may also be used. Between each pair of fiber spans 110A-C, system 100 has a bidirectional gain block 120, e.g., gain block 120A between fiber spans 110A and 110B. Each gain block 120A-B includes a separate unidirectional optical amplifier (OA) 106 for each direction configured to amplify optical signals travelling in that direction. Optical routing elements, i.e., circulators 108, are included in gain blocks 120 to direct optical signals between the corresponding OA 106 and fiber span 110. Furthermore, additional OAs 106 and circulators 108 are positioned next to multiplexers 104E-W and demultiplexers 112E-W.
FIG. 2 illustrates a representative channel allocation plan that may be used in system 100. Odd-numbered channels (e.g., channels 1, 3, 5, and 7) and even-numbered channels (e.g., channels 0, 2, 4, and 6) are interleaved and used for signal transmission in the W/E and E/W directions, respectively. The wavelengths assigned to the channels (i.e., xcex1, xcex3, xcex5, and xcex7 for the odd-numbered channels and xcex0, xcex2, xcex4, and xcex6 for the even-numbered channels) are desirably spaced, preferably equidistantly, with wavelength increasing monotonically with the channel number. Channel spacing is chosen to keep inter-channel crosstalk below a specified threshold.
One known source of inter-channel crosstalk in WDM systems (unidirectional or bidirectional) is four-wave mixing (FWM). Due to the FWM, a pair of different co-propagating (i.e., propagating in the same direction) optical signals generates (mixes into) a third co-propagating optical signal having a frequency (or wavelength) related to but different from those of the pair. If the wavelength of the third optical signal corresponds to that of a fourth co-propagating optical signal used for data transmission, the third signal will interfere with data transmission giving rise to inter-channel crosstalk for the fourth optical signal. Due to the phase-matching condition violation, counter-propagating (i.e., propagating in the opposite direction) optical signals do not mix via FWM. Using this fact, a bidirectional interleaved WDM system, such as system 100, can be configured with twice as many communication channels per optical fiber within the same wavelength range as its unidirectional counterpart without significantly increasing the amount of inter-channel crosstalk.
However, a problem that arises in a bidirectional system is that a signal propagating in a given direction will inevitably experience factors that result in some reflection of the signal that will cause part of it to travel in a direction opposite to its original direction of propagation and so to affect deleteriously the signals of channels launched to propagate in such opposite direction. Accordingly, design of bidirectional interleaved WDM systems requires special consideration of this problem, particularly in the construction of optical amplifiers.
Certain embodiments of the present invention provide an interleaving combiner (ILC) that may be used to implement hybrid amplification in a bidirectional interleaved wavelength-division multiplexing (WDM) system. The ILC implements a composite pump/signal combining and interleaving filtering function in a single topology characterized by relatively low insertion losses. In one form, the ILC is a four-port device that can be configured to (i) route bidirectional optical signals corresponding to different communication channels to and from unidirectional lumped optical amplifiers and (ii) combine those signals with a pump signal for distributed amplification in a fiber span between different lumped amplifiers. The ILC provides relatively high rejection for the pump signal along optical paths different from the intended one and relatively low in- and out-of-band crosstalk for the communication channels. The ILC may be implemented using a Gires-Tournois interferometer.
According to one embodiment, the present invention is an interleaving combiner (ILC), comprising at least four ports, wherein: the ILC is capable of routing a first set of one or more optical signals between a first port and a second port of the ILC; the ILC is capable of routing a second set of one or more optical signals between the first port and a third port of the ILC; and the ILC is capable of routing an optical pump signal between the first port and a fourth port of the ILC.
According to another embodiment, the present invention is an amplifier for use in a bidirectional wavelength-division multiplexing system, the amplifier comprising: (a) a first and a second ILC, each ILC comprising at least four ports, wherein: each ILC is capable of routing a first set of one or more optical signals between a first port and a second port of the ILC; each ILC is capable of routing a second set of one or more optical signals between the first port and a third port of the ILC; and each ILC is capable of routing an optical pump signal between the first port and a fourth port of the ILC; (b) a first optical amplifier (OA) configured between the second ports of the first and second ILCs; (c) a second OA configured between the third ports of the first and second ILCs, wherein each of the first and second ILCs is configured to receive the optical pump signal.
According to yet another embodiment, the present invention is an ILC, comprising: (A) first, second, and third ports, wherein the ILC is designed to: route a first set of one or more optical signals between the first port and the second port of the ILC; route a second set of one or more optical signals between the first port and the third port of the ILC; attenuate optical signals corresponding to the second set between the first and second ports; and attenuate optical signals corresponding to the first set between the first and third ports; and (B) a fourth port, wherein the ILC is designed to: route an optical pump signal between the first port and the fourth port of the ILC; and attenuate the optical pump signal for any optical path different from an optical path corresponding to the first and fourth ports.